Power electronics are becoming increasingly important to applications such as power distribution, transportation, spacecraft, medical devices, and weapons. Modern power electronics use solid state switching devices to convert AC to DC and to change voltages and waveforms. The electronics are most often computer programmable and can be used to rapidly change the type of power delivered in response to changing demands. On large and medium size transportation platforms such as ships, submarines, aircraft, and hybrid electric vehicles, electric drive systems are replacing direct mechanical linkages, such as drive shafts, resulting in highly efficient and adaptable propulsion systems. Advanced power electronics are also making it possible to reroute power on demand from electric motors to weapon systems or around damaged zones resulting in highly adaptable, fault tolerant mobile platforms. A common theme among all these systems is the need to reduce size and weight so that power conditioning modules of increasing capacity and functionality can be incorporated into a greater range of applications. Of all the components used to make power conditioning modules, capacitors usually take up the largest volume and often the greatest weight.
Capacitors are also a key component in pulsed power circuits used to deliver very large amounts of energy in a very short time. Pulsed power circuit applications include medical defibrillators, directed energy tools and weapons including lasers, RF, microwave and X-ray, detonators, electromagnetic armor and electromagnetic launchers. In all these systems, energy is stored in capacitor banks before being rapidly discharged into a circuit that performs a function. The main limitation to improving these systems either by delivering more energy or by reducing their size, particularly on mobile platforms, is the capacity and size of the energy storage capacitors.
For both power conditioning and pulse power applications, the amount of electrical energy per unit volume or weight, the energy density, that can be stored in the capacitor is a critical factor in determining the size of the power electronic system. The energy density is determined by the component's capacitance and maximum safe operating voltage. The greater a capacitor's energy density the more energy it can deliver to a circuit or the smaller the capacitor can be used to deliver the same energy relative to a lower energy density component.
Capacitors are characterized by the dielectric constant of the dielectric material used to store energy, their losses which generate heat on discharge, the voltage levels that can be applied to them, their operating temperature ranges, and the ranges of capacitance levels that can be achieved. At the most basic level, the structure of a capacitor consists of a slab of dielectric material between two electrically conducting plates. Applying a voltage across the plates causes energy to be stored in the dielectric material. The capacitance of the capacitor is determined by:
                    C        =                              k            ⁢                                                  ⁢                          ɛ              0                        ⁢            A                    t                                    (        1        )            Where C is the capacitance in Farads, k is the relative permittivity (dielectric constant) of the dielectric material, ∈0 is the permittivity of free space (8.854×10−12 F/m), A is the area of dielectric covered by the conducting plates, and t is the thickness of the dielectric. The energy stored in the dielectric is:U=∫0EEdP=∈0∫0EE∈(E)dE  (2)Where U is the stored energy in Joules, E is the electric field applied to the dielectric, dP is the change in polarization induced in the dielectric by the applied field and ∈(E) is the electric field dependent permittivity (k(E)=∈(E)/∈0). For most dielectrics the permittivity and dielectric constant are not field dependent, therefore, the stored energy reduces to:
                    U        =                              1            2                    ⁢                      ɛɛ            0                    ⁢                      E            2                                              (        3        )            Or for a capacitor:
                    U        =                              1            2                    ⁢                      CV            2                                              (        4        )            
Therefore, as can be seen by these relationships, more energy is stored in a dielectric with a high permittivity (high k), in a capacitor with a high capacitance (high C), and in a dielectric or capacitor that has a high breakdown strength allowing the application of high E or V. The various capacitor technologies have differing fabrication methods and differing quality of dielectric and connecting electrodes. Capacitors that can be made with thin, high quality dielectric and low resistivity electrodes can be operated with a high electric field across the dielectric resulting in high voltage and therefore high energy density. Capacitors with low loss dielectric and low resistivity conductors can be discharged very quickly and can operate continuously at high frequency and high power.
Polymer and paper capacitors are made with dielectrics that have a low dielectric constant (k), on the order of approximately 2 to 4, but a very high breakdown strength. High energy density is thus achieved in these capacitors by operating them at high voltage (high electric field across the polymer). The capacitors are made by rolling sheets of dielectric with metal foils between them to serve as electrodes or rolling sheets that have been coated with a thin film of metal. The sheets can be made very thin and can be rolled into large capacitors (100's μF). Polymers also exhibit low dielectric losses. As such they are the state-of-the-art for both power conditioning and pulse power applications. Pulsed power polymer capacitors have energy densities approaching 2 J/cm3. They can also be made to fail gracefully instead of catastrophically. However, they have low melting temperatures so they cannot usually be used much higher than 100° C. Also for power conditioning applications, where the capacitors are subjected to high frequency signals for long periods of time, the operating voltages must be significantly derated from what would be used in pulse power applications. Therefore, the effective energy density of polymer (and paper) capacitors in power conditioning applications is only 0.2 to as low as 0.02 J/cm3.
Higher energy densities are needed to reduce the size of polymer capacitors in both power conditioning and pulse power circuits. However, the quality of current polymer films is very high, and there are few areas of polymer capacitor manufacturing technology that can be improved to further increase breakdown strength. There have been many recent attempts to improve polymers by increasing dielectric constant, but this often results in reduced temperature ranges and increased loss. Conventional polymer films range from 10 to 20 μm in thickness. Therefore energy density can often be rather low in all but the highest voltage capacitor designs. However, film less that 5 μm and even less than 1 μm is becoming available so that high energy density can, in principal, be achieved at low voltage.
Electrolytic capacitors consist of a dielectric film that is grown on a metal anode by electrochemical reactions. The dielectric can be aluminum oxide (Al2O3−k=8); tantalum oxide (Ta2O5−K=25), or niobium oxide (Nb2O5). The anode that the film is grown on has a very high surface area so that the “A” term in equation (1) is very large resulting in a large capacitance (can be as high as 10,000's μF). For aluminum electrolytics the film is grown on an etched sheet of aluminum metal. For Ta and Nb electrolytics the film is grown on a porous, sintered sponge of Ta or Nb metal particles. In both cases the cathode is in contact with an electrolyte which is a liquid for aluminum capacitors and a solid or liquid for Ta and Nb capacitors. For aluminum capacitors the film is rolled and placed in a cylindrical package, which is then back-filled with electrolyte. Electrolytics have a high capacitance due to the relatively high k of the dielectric and very high surface area. The films are very thin therefore even at small voltages the electric fields are very high. As a result these capacitors have an energy density nearing 1 J/cm3. They can be used for both pulse and power conditioning applications. Unfortunately, the electrolyte cathode results in very high losses, and a lot of capacitors must be connected in parallel to prevent overheating in high frequency power conditioning. For pulse power applications the losses limit how fast the capacitor can be discharged. They are also limited in temperature to about 80 to 100° C. due to conduction effects and reliability concerns, and aluminum capacitors can not be made to operate above 500V due to limitations in how thick the dielectric layer can be. Ta and Nb capacitors are limited to even lower voltages. Therefore, many capacitors must be connected in series for high voltage circuits.
Ceramic and glass capacitors are used in a broad variety of applications including very compact, severe environment, and very high power electronics. Ceramic formulations cover a wide range of dielectric constants from approximately 5 for glass capacitors to about 40 for temperature stable compositions, and from about 2000 to over about 10,000 for large capacity or very compact applications. They can also be made to operate at very high voltages (>10,000 V). The largest ceramic capacitors are on the order of 10's of μF. However, the breakdown field of ceramic is generally much less than polymers therefore the energy densities of these components are rarely higher than 0.2 J/cm3. In addition ceramics with high dielectric constants exhibit field dependence so that as field and voltage is increased the dielectric constant decreases. This results in lower energy density than would be expected from low field measurements of capacitance. However, unlike polymers this energy density can be achieved in both pulse and power conditioning applications because ceramics have relatively low losses and much greater temperature stability than polymers. Ceramics are less affected by heat generation during AC drive than the other capacitor technologies. Ceramics find wide use in power conditioning applications, but are used less in pulse power applications because they are more expensive than polymers for large capacitor banks and they tend to fail catastrophically. The layers in ceramic capacitors can be made very thin with 1 to 2 μm being the current state-of-the-art. Therefore, the maximum energy density for these dielectrics can be achieved at low voltages.
Mica capacitors are used for high temperature and high voltage applications. Unfortunately their energy densities are very low because their dielectric, mica paper, has a low dielectric constant, and only a moderate breakdown strength. In addition mica paper can only be manufactured in relatively thick sheets. This is good for high voltage capacitors, but it also results in very low capacitance and since the electric fields on the sheets tend to be quite low the energy density is generally <0.1 J/cm3.
Super and Ultra capacitors are electrochemical devices that use carbon fiber to create extremely high surface area electrodes which result in very high capacitance values. Their energy densities can be as high as 10 J/cm3 and capacitance values range from mF to Farads. However, operating voltages are limited to only about 4V requiring extensive series stacking for higher voltage power applications. They have very high losses limiting discharge times to the order of seconds. They also have temperature limitations similar to polymers and electrolytecs. They find use in moderate to large scale, low frequency power conditioning applications, but not in pulse power and not in high frequency power conditioning the operating regime of state-of-the-art solid state power converter technology.
State-of-the-art and future power conditioning and pulse power systems require much smaller capacitors than are currently available. Size reductions of a factor of 2 or better are required to enable many applications such as electric vehicles, electric ships and aircraft, and pulse power weapons and electromagnetic armor on mobile platforms. These capacitors need to operate efficiently at frequencies ranging from 1 kHz to greater than 500 kHz and they must be able to discharge repeatably in less than 1 μs. They also need to operate over a broad temperature range from <−55° C. to >200° C. No existing capacitor technology can meet these exacting requirements. Advanced capacitor technologies in development to go beyond the state-of-the-art include:
1) New polymer dielectrics with dielectric constants >50 and/or expanded temperature capabilities
2) Thin film dielectrics such as diamond like carbon (DLC) and aluminum oxynitride (ALON)
3) Super capacitors made with carbon nanotubes.
However, these developing technologies have encountered manufacturing and cost difficulties that have prevented these technologies from meeting capacitor requirement demands for pulse power and high frequency power conditioning.
Of the above technologies, only polymers and thin film dielectrics can meet the needs for high frequency, fast discharge power electronics. Polymers can be made to have high energy densities and some have been developed to operate at elevated temperature, but to date none exhibit high energy density over a broad temperature range. Thin films dielectrics have the potential to meet the needs of emerging power electronics technology, but to date, these capacitors must be made by depositing a film of dielectric on a carrier substrate that is incorporated into the structure of the capacitor. This fills capacitor volume with material that does not store energy greatly reducing the capacitor energy density. In addition, capacitors made with thin film technology are likely to be very expensive due the long times and expensive equipment needed to deposit films. Making super and ultra capacitors with carbon nanotubes increases energy density, nanotube capacitors still suffer from the same high losses, slow discharge times, low voltage limits and narrow temperature limits as conventional super and ultra capacitors.
Antiferroelectrics based on the (Pb,La)(Zr,Ti)O3 (PLZT) material system have been proposed previously for use in energy storage capacitors in U.S. Pat. No. 4,027,209. Materials in this composition family can exhibit very high energy densities (on the order of 12 J/cm3) although these high values come at the cost of high loss (˜15%) and high required electric field. Both factors make it difficult to use these materials in AC power applications with operating voltages in the 500 to 2000V range. In addition, PLZT materials with high Zr content have high vapor pressures making them difficult to process.
What is needed is a high energy density capacitor formed of a high energy density, broad temperature range dielectric material that can be fabricated using conventional or nearly conventional manufacturing methods, which does not suffer from the drawbacks of the prior art.